1. Field of the Invention
The present invention relates to electrographic reproduction apparatus and method and more particularly to an improved approach for duplex reproduction (i.e., the forming of images on both sides of a copy sheet).
2. Description of the Prior Art
Electrographic copying is currently a highly preferred technique for document reporduction. However, until recently most electrographic copying apparatus did not utilize both sides of a copy sheet, i.e., formed images only on one side. As is apparent this was wasteful, both of the copy paper and the storage space required to accommodate reproduced documents. Additionally, this characteristic caused inconvenience when copying originals which had information on both sides as do most printed publications. For example, the operator must manually turn over the original to copy the other side. In simplex (one-sided) copying of multi-page, two-sided originals complications evolve in maintaining the proper page sequence.
To obviate the disadvantages of simplex copying, various approaches have been developed, two somewhat straightforward approaches being the first suggested. One of those provides two complete electrophotographic systems which operate concurrently and respectively form images on opposite copy sheet sides. Another feeds a series of duplex originals and copy sheets past the copy station in first and second passes with the operator turning both stacks over between passes. Both techniques have obvious disadvantages, the two-machine approach being extremely costly in hardware and the two-pass technique being time consuming and requiring operator interplay.
Thus, more sophisticated approaches have evolved. For example, in one such approach both sides of a duplex original are concurrently exposed onto succesive image areas of the photoconductor. The developed toner image of the first document side is transferred to a drum adjacent the photoconductor path and then transferred to one side of a copy sheet during transfer of the subsequent toner image from the photoconductor to the other side of the copy sheet. This approach obviates the disadvantages outlined above; however, the concurrent electrostatic transfer of two toner images to opposite sides of a copy sheet presents a difficult procedural step to perform reliably with high quality.
Another approach disclosed in U.S. Pat. No. 3,672,765 likewise provides for concurrent exposure of the two sides of the duplex document onto successive image areas of a photoconductor. However, instead of effecting a simultaneous transfer of the two developed images to the copy sheet, this technique utilizes sequential transfers with an intervening turnover of the copy sheet. While this approach avoids the difficult electrostatic transfer problems, it introduces a constraint on machine copying rate. Specifically, a time interval must be allowed, between successive transfers of the duplex original images, for the mechanical-turnover of the copy sheet. In addition, two transfer stations are utilized.